Re: grounding NST's

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: FIFTYGUY@xxxxxxx 

In a message dated 3/21/06 8:04:53 PM Eastern Standard Time, 
tesla@xxxxxxxxxx writes:

> I would also put a line filter on the primary side
>of a MOT or NST and a RFI choke (low impedance to 60Hz, high
>impedance to RF) on the ground between the case of the NST and a
>Terry filter on the HV side.

    Dave, what kind of filter would you feel appropriate for the RFI 
choke here? Would a ferrite bead/ring type as used on power cords, 
with a couple turns of the wire connecting NST case and Terry/RF 
ground be sufficient? Or are you thinking a larger air-core coil? 
What value range?

>Grounding/bonding is a critical exercise to industrial equipment; people have
>DIED by having defective safety ground bonds.

    Only because they touched something that was "plugged in" and 
that they thought would be at safe potential while they were in 
contact with it. Doesn't solve the problem with things still plugged 
or hardwired in, or that store energy , such as caps or batteries. 
And since we don't touch the high voltage side of our Tesla coils 
while they are plugged in or while the caps are energized, this 
shouldn't be an issue with NST cases unless the NST is located at the 
control panel.
    So let me throw this *new* question out there:
    Even if you put the NST at the coil proper (in "high voltage 
land"), you still have to run power (LV) wires to it. So how do you 
ground things in case of a secondary strike to these LV wires?
    I'm running my LV wires in a flexible metal gooseneck conduit, 
so a secondary strike will hit the conduit instead of the power 
wires. These power wires include the "hot" to the NST from the 
variac, the "hot" to run the SRSG from the phase control, the "hot" 
to run the gap cooling fans, and a neutral common to all three. 
Obviously, a secondary strike to any of these would be bad. But the 
conduit will be a relatively large piece of tubular metal laying on 
the floor, so I'm hoping it will have low impedance to RF ground and 
thus protect the wiring inside.
    So far so good? But how do we bond it to ground? After all, a 
short of any of the hots to the conduit will make the whole thing 
hot, and a long piece of flexible metal conduit on a concrete floor 
isn't necessarily going to trip any breakers or blow fuses to 
indicate the fault. And it isn't safe, either, since the conduit is 
connected at the panel end and therefore accessible during normal 
(plugged in) operation. So we can't leave the conduit floating.
    One end (at the coil?) to RF ground? Remember, the theory is to 
have low impedance to RF ground. But we have other concerns as well. 
So other end (at the control panel?) to house ground? But then what 
if it takes a streamer hit?
    And we can't bond it to *both* house and RF ground, because that 
would be like tying both together. Maybe if the conduit was itself an RF choke?
    Perhaps the conduit should be nonmetallic up to the point of the 
maximum expected reach of RF and streamer hits, but metallic and tied 
to RF ground in the vicinity of the coil?

-Phil LaBudde